OSD: rejigger OSD::shutdown

Reorder teardown:
- pgs
- queues/threadpools
- persist superblock
- filestore
- timers
- messengers

Signed-off-by: Samuel Just <sam.just@inktank.com>
ceph_osd: clear client_throttler prior to putting g_ceph_context

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileJournal: quieter debugging on journal scanning

Signed-off-by: Samuel Just <sam.just@inktank.com>
not sure about this one